The objective of the proposed research is to investigate the relationship between steroid hormone action at the cellular and molecular levels within the central nervous system and the hormonal regulation of behavior. More specifically, the experiments to be described are concerned with the role of neural receptors for one class of steroid hormones, the progestins, in the mediation of female sexual behavior in rodents. After a sufficient period (24-48 hr) of estrogen priming, an injection of progesterone synergizes with estrogen to induce high levels of sexual receptivity in female rodents. This same period of estrogen priming also induces the synthesis of progestin receptors in the hypothalamus of rats and guinea pigs, a brain region known to be intimately involved in the activation of female sexual behavior. The research outlined in this proposal will examine the interactions of progestins with the neural receptor system to assess whether (a) estrogen induction of progestin receptors, (b) progestin-specific binding to cytosol receptors, (c) depletion of cytosol receptors and translocation to the nucleus, (d) binding of the nuclear progestin-receptor complex to the chromatin material, and (e) progestin induction of specific protein synthesis are positively correlated with the capacity of various progestins to facilitate estrous behavior in estrogen-primed rodents of several species. The experiments will rigorously test the relevance of steroid-receptor interactions to progestin activation of a specific brain response and will thus provide information regarding potential molecular mechanisms of steroid regulation of neural functions.